Secondhand smoke exposure among never-smoking adolescents in Wuhan, China

Without smoke-free legislation in Wuhan, China, we investigated secondhand smoke (SHS) exposure at home, school, and public places for never-smoking school-going adolescents in 2019. A cross-sectional study was carried out within the Global Youth Tobacco Survey (GYTS) framework. Weighted univariate, bivariate and multivariable analyses were conducted. The prevalence of SHS exposure among never-smoking adolescents at home, school and public places was 25.7%, 31.9% and 48.9%, respectively. Multivariable logistic regression analysis showed that parents smoking, peers smoking and observing teachers smoking in school were all significantly related to a higher probability of SHS exposure at home, school, and public places. Never-smoking adolescents who had smoking parents had 14 times (adjusted odds ratio [aOR], 14.00; 95% confidence interval [CI], 11.37–17.24) higher odds of SHS exposure at home; Never-smoking adolescents who observed teachers smoking in school had about 10 (aOR = 9.76; 95% CI = 7.13–13.36) and four times (aOR = 3.55; 95% CI = 2.77–4.55) higher odds of SHS exposure in school and public places, respectively. Adopting comprehensive smoke-free legislation in public places and smoke-free home rules and implementing and supervising smoke-free school policies may further reduce SHS exposure among adolescents.


Scientific Reports
| (2022) 12:14209 | https://doi.org/10.1038/s41598-022-18612-y www.nature.com/scientificreports/ home and in public places, respectively 15 . The data among never-smoking adolescents were 39.1% and 49.5%, respectively 16 . Parents' or peers' tobacco use, knowledge of the harms of SHS, and positive attitudes toward the smoking ban were related to higher exposure to SHS 16 . The National Youth Tobacco Survey in China (NYTS) uses the standard sampling methods, questionnaires and data analysis methods of the GYTS 17 , which showed suitability by the reliability and validity analysis 18 . The NYTS was first conducted in the fall of 2013 and again in the fall of 2019, providing representative data of all junior high school students in the 31 provinces of China. However, there are few articles about China based on the NYTS between 2000 and 2020, and the topic was about smoking prevalence, electronic cigarettes, smoking experimentation, and comparison of tobacco use 17,[19][20][21] . There is limited data on exposure to SHS among adolescents in China, and there is no data in Wuhan either. Wuhan is the capital of Hubei and the core city of central China, with more than 12 million permanent residents and 200,000 junior high school students 22 . In conjunction with the NYTS in 2019, the Wuhan Youth Tobacco Survey (YTS) was developed to provide the data necessary to estimate adolescents' tobacco use comprehensively. This study aims to provide representative baseline data on SHS exposure among never-smoking adolescents in Wuhan to guide subsequent research and tobacco control efforts. To our knowledge, it was the first city-level investigation into YTS in Wuhan.

Methods
Sampling design. The Wuhan YTS was a cross-sectional, school-based survey of junior high school students using the GYTS standardized protocol and questionnaire. The universe for the survey consisted of junior high school students in grades 7-9 in all public and private schools in 15 districts of Wuhan. The universe of this paper was junior high school students who had never smoked. The sampling frame consisted of primary sampling units (PSUs) made up of schools that can supply a whole complement of students in grades 7-9. Schools with fewer than 40 students were not included in the sampling frame. A representative sample was generated using a two-stage cluster sample design. In the first stage, 45 PSUs (overall 277 PSUs) were selected from 15 administrative districts using the probability proportional to size (PPS) method. In the second stage, one class was selected from each grade in PSU with the simple random sampling method, and a total of 135 classes were selected. All students in the selected class were invited to participate in the survey. The sampling frame, obtained from Wuhan Municipal Education Bureau, contained all school information, including the number of students, school type and other contact information.
Survey instrument. The 2019 NYTS survey instrument included 45 questions. The first four questions on the survey collected adolescents' demographic characteristics, and the rest measured a comprehensive set of tobacco-related topics. Specific topics included: tobacco product use, cessation attempts, knowledge of and attitudes toward tobacco use, SHS exposure, harm perceptions, and exposure to pro-and anti-tobacco advertising and media 13 . There are all single-choice questions and no skip mode. The questionnaire was prepared in Chinese, China's common and official language. The self-administered questionnaire was conducted using traditional paper-and-pencil (PAPI). One class period (about 40 min) was given to students to complete the questionnaire. Students absent on the survey day were not asked to complete the questionnaire after returning to school. Survey administration. The research manual of GYTS 23 , which includes detailed management procedures, was followed strictly. The data collector and investigation coordinator training was conducted on September 12-14, 2019, to ensure the survey protocol and procedures would be identical. Survey administration in the schools began on September 26, 2019, immediately after the training, and continued until November 21, 2019. Students participated in the survey anonymously and voluntarily. The survey did not collect personal information, such as name, ID number or other identifying information, and school staff were not present during the survey. Informed consent was obtained from all participants and parents or legal guardians of participants younger than 16. The study was conducted following the Declaration of Helsinki, and the processes, security elements, and sampling design were reviewed and approved by Wuhan CDC Institutional Review Board (IRB).

Variables.
As school-going adolescents spend much time at home and school [24][25][26] , they also inevitably go to public places. The dependent variables were SHS exposure at home, school and public places 6,27,28 . Similar to earlier studies 6,26,[29][30][31] , the following independent variables potentially related to the dependent variables were included. These variables were gender, grade, knowledge about the harm of SHS, parent or peer smoking behaviour, whether the adolescents observed teachers smoking in school, school type, disposable income, exposure to anti-smoking media and tobacco advertising and promotion. The detailed coding of the survey items is shown in Table 1.
Response rates. The survey's response rate was explained at the school and student levels. At the school level, 44 of the 45 schools selected (97.8%) participated in the survey. The remaining one school was considered to refuse because of losing instructional time. At the student level, 6,126 eligible students were invited to participate in the survey, of which 6,069 did so, and the response rate of students was 99.1%. The remaining 57 students did not participate because they were out of school on the survey day or unwilling to participate. When the response rates of students and schools were combined, the overall response rate was 96.9%, thus considered sufficient for weighting purposes. Data analysis. The  www.nature.com/scientificreports/ were created to ensure accuracy and eliminate internal inconsistencies. The sampling weight was considered since the survey adopted a multistage cluster sampling procedure. The data were weighted to represent the population of junior high school students in Wuhan by a three-step process: (1) calculation of the base weight (school and class levels), (2) nonresponse adjustment (school and student levels), and (3) post-stratification calibration adjustment of sample totals to the known population totals 15 . Rao-Scott chi-square tests were used for differences in characteristics among subgroups in bivariate analysis. Multivariable logistic regression analysis was conducted to identify the factors related to SHS exposure. Statistical inferences were assessed by a two-sided 5% significance level. Data analyses were conducted with SAS software version 9.4 (SAS Institute Inc. Cary, NC).

Results
Sample characteristics. A total of 5,632 never-smoking adolescents aged about 12-15 (99.2% were in the 12-15 age group) were included in the study, representing 205,552 never-smoking adolescents in Wuhan. About half of adolescents (53.3%) who had never smoked were male, and students in grades 7, 8 and 9 each accounted for about one-third. More than two-fifths (41.7%) of adolescents who had never smoked had smoking parents, and about one-ninth (10.6%) had smoking peers. Moreover, 97.6% had knowledge about the harm of SHS exposure, and 29.6% observed teachers smoking in school (Table 2).
Bivariate analysis. Overall, 25.7%, 31.9% and 48.9% of the school-going adolescents who had never smoked were exposed to SHS at home, school and public places, respectively. Table 3 presents the bivariate analysis of potential factors related to SHS exposure. Of the ten variables considered, all but three (gender, grade, and school type), one (grade), and two (gender and grade) were statistically significantly related to SHS exposure at home, school, and public places, respectively (P < 0.05). Table 1. Question-wording, response options and coding. "No" responses were coded as "0" and "Yes" responses as "1" for the multivariable logistic regression model. SHS secondhand smoke, CNY Chinese Yuan, 1 United States dollar = 6.7 Chinese yuan. *Set "I don't know" as missing values.

Survey items
Original response options Dichotomized measure

Item used to obtain never-smoking adolescents
Smoking status Have you ever tried or experimented with cigarette smoking, even one or two puffs? No; Yes No = never smoker;Yes = ever smoker

Dependent variable
Exposure to SHS at home During the past seven days, how many days did someone smoke tobacco products in your home while you were there? 0 day; 1-2 days; 3-5 days; 6-9 days; 7 days No = 0 day; Yes = ≥ 1 days

Exposure to SHS in public places
During the past seven days, how many days did you smell the smoke from someone smoking tobacco products in an indoor or outdoor public place? During the past seven days, how many days did you ride in a vehicle when someone was smoking a tobacco product? www.nature.com/scientificreports/ Multivariable logistic regression analysis. While gender, grade, and school type were not significantly related to SHS exposure, these variables were retained in multivariable logistic regression models for possible confounding 13 . The multivariable logistic regression analysis showing how individuals' characteristics were related to SHS at home, school, and public places, respectively, among never-smoking adolescents, was shown in Table 4. The likelihood-ratio tests of the logistic regression were performed, and the results showed that all P < 0.001. It can be considered that the logistic regression equations were statistically significant. When considering the risk factors for SHS at home, We found the most increased odds of SHS prevalence among never-smoking adolescents who had smoking parents (adjusted odds ratio [aOR], 14.00; 95% confidence interval [CI], 11.37-17.24), followed by those who had knowledge about the harm of SHS (aOR = 2.77; 95% CI = 1.38-5.56), observed teachers smoking in school (aOR = 1.66; 95% CI = 1.32-2.08), had more than 50 Chinese Yuan (CNY) disposable income per week (aOR = 1.64; 95% CI = 1.20-2.23), had smoking peers (aOR = 1.47; 95% CI = 1.02-2.14), attended private school (aOR = 1.40; 95% CI = 1.17-1.68), and were in grade 7 (aOR = 1.47; 95% CI = 1.10-1.96) or grade 8 (aOR = 1.39; 95% CI = 1.14-1.70).

Discussion
Exposure to tobacco smoke remains a global health problem. Monitoring tobacco use is critical to combating the tobacco epidemic and assessing the implementation of FCTC and MPOWER measures 2 . Exploring the potentially related factors of local SHS exposure is significant for formulating and implementing tobacco control policies and SHS exposure intervention measures. To the best of our knowledge, this is the first study to estimate the prevalence and related factors of SHS exposure in a representative sample of never-smoking adolescents in Wuhan, China. The survey showed that SHS exposure was prevalent among never-smoking adolescents in Wuhan, China. Approximately one-fourth (25.7%), one-third (31.9%), and one-half (48.9%) of adolescents who had never smoked were exposed to SHS at home, school and public places, respectively. The NYTS showed that in China, 44.4%, 57.2% and 58.3% of adolescents who had never smoked were exposed to SHS at home, school and Table 3. Prevalence estimates of secondhand smoke (SHS) exposure among never-smoking adolescents (Weighted). SHS secondhand smoke, CNY Chinese Yuan, 1 United States dollar = 6.7 Chinese yuan. *Unweighted frequencies, weighted percentages.  32 . According to the latest surveys in 142 countries, the prevalence of SHS exposure at home and in public places was 33.1% and 57.6%, respectively 6 . According to previous studies, differences in the prevalence of SHS exposure can be attributed to regional differences in demographic characteristics, economic levels, lifestyles, and smoking prevalence 26,29,33 . Previous studies have shown that SHS exposure can be a proxy for social pressures to smoke from peers, teachers, or parents 29,34 . Similar to these studies, we found in the multivariable analysis that parent or peer smoking and observing teachers' smoking in school were significantly related to adolescents' exposure to SHS. The strongest determinants related to exposure to SHS at home among adolescents who had never smoked were parent smoking, consistent with earlier studies, and the strongest determinants in school and public places were observing teachers smoking in school 16,29,33 . There may be the following reasons. First, previous studies showed that SHS exposure was positively related to local tobacco control policies 16 . Currently, there are no national-level smoke-free regulations or laws in China 35 . For public places, the Smoking Ban in Public Places of Wuhan was promulgated by Wuhan Municipal People's Government in 2005, stipulating that smoking was prohibited in some indoor public places and school teaching places 36 . However, the Smoking Ban had a low level of legal effect and a limited range of tobacco control places, the punishment measures were light, and the principal enforcement department was not transparent 35 . For households, no smoke-free regulations at home were adopted in Wuhan, and 62.2% of households had no voluntary smoke-free home rules 37 . Second, school-going adolescents usually spend much time at home and school 5,26 , but they usually lack the ability to avoid SHS as they have limited say in smoking, which may further make them tolerant of smoking and SHS. At last, the smoking behaviour of parents, www.nature.com/scientificreports/ peers and teachers will affect adolescents' attitudes towards smoking favourably, and adolescents would tolerate, agree, and even imitate this behaviour. They may also not deliberately or actively avoid SHS in public places. There was evidence that reducing adult smoking can help reduce adolescents' exposure to SHS to a large extent 2,38 . The way to reduce adult smoking may be to adopt further and widely implement the WHO FCTC and its guidelines, such as raising tobacco taxes, adopting comprehensive smoke-free legislation in public places, completely banning TAPS, and providing effective programs to assist smokers to quit 2,39 . However, the smoke-free regulations or laws cannot be extended to private residences and social life 29 . Therefore, abstaining from smoking at home mainly relied on non-regulatory measures. Studies have shown that public education campaigns, community intervention, social norms, and smoke-free home rules can substantially reduce SHS exposure at home 4,6,29,40 . Corresponding strategies, such as educating parents about the risks of smoking, SHS exposure and the benefits of smoke-free home rules, should be implemented to motivate them to ban smoking at home voluntarily 30 . As for teachers' smoking in schools, the Ministry of Education of the people's Republic of China posted a notice on the smoke-free school in 2014 41 , and the effect seemed unsatisfactory. Relevant government departments need to strengthen the implementation and supervision of the smoke-free policy and strengthen training, publicity and education to achieve a truly 100% smoke-free school. Furthermore, great efforts should be made to monitor and evaluate the effects of the above measures to sustain broad public support and formulate best practices 42 .
In the multivariable analysis, the negative relationship between SHS exposure (especially at home and school) and adolescents' knowledge about the harm of SHS was consistent with the results reported by Veeranki SP et al. 16 and Mamudu HM et al 4 . The universe of the survey was adolescents, who were vulnerable groups and had limited ability to avoid smoking environment, which meant that knowledge about the harm of SHS has a limited impact on behaviour 16 . We also found that exposure to SHS in school and public places were significantly higher for adolescents exposed to tobacco advertising and promotion, consistent with earlier studies 43,44 . Exposure to tobacco advertising and promotion may shift adolescents' attitudes toward smoking favourably by shaping adolescents' curiosity about smoking, perceived norms, and perceptions about the benefits of smoking, thus making adolescents accept and tolerate smoking and SHS 45,46 . According to WHO FCTC, there should be a broad ban on all tobacco advertising, promotion, and sponsorship 2 . In response to the WHO FCTC, the Advertising Law of the People's Republic of China was revised in September 2015, in which Article 22 stipulates that tobacco advertisements in the mass media, public places, public transport, and outdoors are entirely prohibited, and any form of tobacco advertisements to adolescents is prohibited. However, a study conducted by the Chinese Association on Tobacco Control reported that tobacco advertisements were found in up to 64.9% of tobacco retail outlets in China, and up to 57.6% of tobacco retail outlets did not have any signs prohibiting the sale of cigarettes to adolescents 47 . The WHO FCTC should be strictly implemented, the enforcement of the Advertising Law should be strengthened, and the supervision and enforcement efforts should be strengthened. In addition, the results indicated that adolescents exposed to anti-smoking media messages were not significantly related to SHS exposure in the study. Based on the three findings above, parents' smoking behaviour, adolescents' exposure to tobacco advertising and promotion and lack of ability to avoid SHS may not only make them tolerant of SHS exposure but also may hinder the effectiveness of efforts to reduce SHS exposure by anti-smoking media messages and knowledge about the harm of SHS 4 .
At last, the multivariable analysis also indicated that adolescents' school type and disposable income might affect SHS exposure. Adolescents' higher disposable income was related to a higher risk of SHS exposure. One explanation may be that adolescents with a higher disposable income had the purchasing ability to eat in restaurants, shop in stores, watch movies in cinemas and play in entertainment venues (such as video arcades, internet bars, and children's parks), which made them more likely to be exposed to SHS in these public places. As adolescents' disposable income may not accurately reflect their family's socioeconomic status 48 , another reasonable explanation was that higher disposable income might indicate less parental engagement and poorer parental supervision 49 , allowing themselves to smoke at home. Compared with adolescents attending public schools, adolescents in private schools were related to a higher risk of exposure to SHS. The poorly implemented and supervised tobacco control policies in private schools and students' continued and passive tolerance of SHS may lead to the phenomenon 6,49 .
This study is subject to several limitations. First, the data were from a cross-sectional survey, and the results should be interpreted cautiously. Second, adolescents' exposure to SHS was assessed based on self-reporting and may be affected by social expectations and reporting biases. Third, there was a lack of information on specific places where adolescents were exposed to SHS (except for school), indicating the need to collect specific information to support local legislation. At last, SHS exposure was assessed using a self-report questionnaire without any objective measurement, such as the levels of nicotine in the air and biomarkers (cotinine and 4-(methylnitrosamino) -1 -( 3-pyridyl) -1 -butanol) in the urine. Adolescents' exposure to SHS may be underestimated, and future studies should adjust them to provide precise estimates of SHS exposure.

Conclusions
With few studies on SHS exposure in China, we studied the prevalence and potential factors related to SHS exposure among never-smoking school-going adolescents in Wuhan, China. The prevalence of SHS exposure among adolescents at home, school and public places was 25.7%, 31.9% and 48.9%, respectively. We substantiated that parent smoking and observing teachers smoking in school were all significantly related to adolescents' exposure to SHS at home, school and in public places. Our findings emphasize the need to strengthen tobacco control policy and program in Wuhan with a particular focus on adolescents, as they are vulnerable. It is crucial to adopt comprehensive smoke-free legislation in public places and smoke-free home rules, implement and www.nature.com/scientificreports/ monitor smoke-free school policy, educate the public about the risk of smoking and SHS exposure, and empower adolescents to insist on their right to smoke-free environments.

Data availability
The data used and/or analyzed in the current study are not publicly available because restrictions apply to the availability of these data. Data are, however, available from the corresponding author on reasonable request and with permission of the Wuhan CDC.